By: Annemieke Aartsma-Rus, Ph.D.

The paper information:

CRISPR-Cas9 In Vivo Gene Editing for Transthyretin Amyloidosis

Julian D Gillmore, Ed Gane, Jorg Taube, Justin Kao, Marianna Fontana, Michael L Maitland, Jessica Seitzer, Daniel O’Connell, Kathryn R Walsh, Kristy Wood, Jonathan Phillips, Yuanxin Xu, Adam Amaral, Adam P Boyd, Jeffrey E Cehelsky, Mark D McKee, Andrew Schiermeier, Olivier Harari, Andrew Murphy, Christos A Kyratsous, Brian Zambrowicz, Randy Soltys, David E Gutstein, John Leonard, Laura Sepp-Lorenzino, David Lebwohl

N Engl J Med . 2021 Jun 26. doi: 10.1056/NEJMoa2107454. Online ahead of print.

Early results of first systemic genome editing trial published!

I doubt the CRISPR/Cas9 genome editing system needs an explanation for this audience. Even my husband, an engineer, knows the basics due to the media attention it continues to receive. Briefly, genome editing allows targeted, permanent changes to be made in the DNA. (Here is a more in-depth overview of CRISPR.) Oligonucleotides, by contrast, target gene transcripts, which have a high turnover. As such, repeated treatment is required for oligonucleotide therapeutics, ranging from weekly to every 6 months, depending on the target transcript and target tissue. Of course, a one-time treatment is preferred and, in theory, genome editing offers this possibility.

Theory has now become reality, according to the first interim results of a clinical trial using systemic gene editing that was published in the New England Journal of Medicine on June 26, 2021. The trial is being conducted in patients with transthyretin amyloidosis (ATTR amyloidosis). This disease is caused by the accumulation of misfolded transthyretin (TTR) protein, that leads to polyneuropathy and/or cardiomyopathy. When untreated, the disease is generally fatal 2-17 years after symptom onset. Reduction in TTR protein production would be expected to result in less misfolding and therefore a slower disease progression. Clinical trials with two oligonucleotide drugs, inotersen (RNase H) and patisiran (siRNA) have shown that reduced TTR protein production indeed is therapeutic. Both drugs have been approved by EMA and FDA. As explained earlier, these oligonucleotide drugs require repeated treatment.

In the new publication, authors used lipid nanoparticles to deliver cas9 RNA and a guide RNA (NTLA-2001) to the liver of six ATTR patients with sensory polyneuropathy. Three patients received a low dose (0.1 mg/kg) and three a high dose (0.3 mg/kg) of NTLA-2001. Treatment was well tolerated by patients, with some mild changes in coagulation measures observed in 5/6 patients. No signs of liver damage were detected.

Serum TTR levels are a biomarker for target engagement and drug efficacy. Patients treated with the low dose had a decrease in serum TTR of 47-56% after 28 days of treatment, while patients treated with the high dose had a decrease of 80-96%. From clinical trials with inotersen and patisiran, it is known that a decrease of ~80% likely will lead to clinically beneficial effects in patients.

These results are very promising but, of course, there are many unknowns with regards to the longevity of reduction and the potential of long-term side effects due to target or off-target gene editing and patients will be carefully monitored for this. Since NTLA-2001 delivers an mRNA for cas9, there is only transient expression of cas9 and therefore off-target editing can only occur in a short time span. With that, the chance of long-term toxicity is lower than when cas9 would be permanently expressed.

I also want to share a word of caution with readers who think that this means systemically delivered genome editing is now possible for all genetic diseases. There is a reason the developers started with ATTR amyloidosis. TTR is produced primarily by the liver (over 99%) and targeting the liver is relatively easy with lipid nanoparticles. Targeting other tissues efficiently, however, is not yet feasible. As an alternative, researchers are utilizing viral vectors to deliver cas9 and guide RNAs. However, this means a much longer expression of cas9. It also risks insertion of viral vectors at the double-stranded break generated by cas9 and, therefore, comes with an increased risk of off-target editing.

Aside from these words of caution, this is groundbreaking work. The first results are very promising and we will learn a lot from this clinical trial as a field*. I hope the follow-up results will be as positive.

*Since genome editing uses guide RNAs, the oligonucleotide therapeutics society considers genome editing as an oligonucleotide therapy.